The present invention relates to a surveying instrument for performing light wave distance measurement by projecting a laser beam to an object to be measured. In particular, the present invention relates to a surveying instrument of non-prism type, which does not use a prism on the object to be measured.
In recent years, a surveying instrument of non-prism type has been propagated, which does not use a prism on the object to be measured but performs the measurement by projecting a laser beam directly to the object to be measured and performs measurement based on a reflection light from the object to be measured.
In the surveying instrument of non-prism type, there is no need to move the prism for each measurement. Thus, the operation can be carried out by a single operator. Further, an arbitrary portion of the object to be measured, e.g. a construction, can be selected as a measuring point. Because the measurement can be made for a multiple of measuring points, 2-dimensional or 3-demensional measurement can be made on the object to be measured.
A surveying instrument of non-prism type is disclosed, for instance, in JP-A-2000-329517.
FIG. 6 represents a case where surveying operation is performed by using a surveying instrument of non-prism type.
In FIG. 6, reference numeral 1 denotes a surveying instrument, and 2 denotes an object to be measured.
When light wave distance measurement is performed, a laser beam (a distance measuring light 3) projected from the surveying instrument has a certain spreading. When the object to be measured is a 3-dimensional object, e.g. a construction such as a building, etc., the construction has ridge lines 4 (edges 4a, 4b, 4c, . . . ) between wall surfaces.
When a measuring point is selected near the edge 4a in the object to be measured 2 or when a measuring point is selected in any manner as desired, the measuring point thus selected (a projecting position of the distance measuring light 3) may be on the ridge line 4, e.g. on the edge 4a. 
When the measuring point is on the edge 4a, the distance measuring light 3 projected from the surveying instrument 1 is split at the edge 4a as shown in FIG. 7. A part of the distance measuring light 3 is reflected by a surface 5 on this side, and the other part of the light is reflected by a surface at a further depth. As a result, the surveying instrument 1 performs measurement by a reflection light from the surface 5 of this side and by the reflection light from the surface 6 at the further depth. Thus, the portion of the edge 4a may not be measured correctly.
In the past, for the confirmation of the measuring point, it has been practiced that a surveying operator has been collimating the measuring point via a telescope 7 mounted on the surveying instrument 1. When the measuring point is on the edge 4a, the measuring point is shifted from the edge 4a. 
However, because the telescope 7 is normally a monocular instrument, a point to be measured is seen as a part of a plane. Because the instrument is of high magnification factor, the edge 4a may not be recognized sometimes. Also, there is a case where measurement is made without being aware that the measuring point is on the edge 4a. In such case, the measured value may be turned to an error.